1. Field of the Invention
The present invention generally relates to a sintered body, and more specifically, it relates to a high-pressure phase boron nitride-based hard and tough sintered body for a cutting tool such as a milling cutter or an end mill, which is improved in wear resistance and chipping resistance.
2. Description of the Background Art
High-pressure phase boron nitride represented by cBN (cubic boron nitride) and/or WBN (wurtzite boron nitride) is the next hardest substance after diamond and has lower reactivity with a ferrous material as compared with diamond. Thus, cBN sintered bodies are applied to various cutting tools.
A hard and tough material is awaited for application to not only a cutting tool but also a wear-resistant component such as a wear-resistant component provided on a sliding part or a wear-resistant component employed for a defensive wall, for example.
Cubic boron nitride is hard to sinter, similarly to diamond, and stable under a high pressure. Therefore, limited sintering conditions of at least 2000.degree. C. and at least 8 GPa are required for sintering only cBN particles. The cBN particles are not bonded to each other under industrial sintering conditions of about 1400.degree. C. and 4 GPa.
In a sintered body consisting of cBN particles and a binder phase, on the other hand, reaction is caused in grain boundaries between the cBN particles and the binder phase. Thus, a cBN sintered body which is excellent in holding power for cBN particles can be obtained.
However, a general cBN sintered body is obtained by mixing cBN powder with binder powder and sintering the mixture, and the cBN particles are not completely homogeneously mixed with the binder. In a partial region of the sintered body, therefore, cBN particles are directly in contact with each other or in contact with a binder phase that hardly reacts with the cBN particles, to result in an unsintered part. Thus, the sintered body is inferior in wear resistance or chipping resistance.
Japanese Patent Laying-Open Nos. 58-58247 (1983), 58-60678 (1983) and 5-186844 (1993) propose typical means for solving the aforementioned problem.
Japanese Patent Laying-Open No. 58-58247 discloses a tough boron nitride-based superhigh pressure sintered material for a cutting or wear-resistant tool which is composed of cubic boron nitride and a binder phase consisting of a boride and a carbide of at least one of Ti, Hf, Zr and Mo so that a layer of the boride having an average thickness of 0.1 to 2 .mu.m encloses the cubic boron nitride.
Japanese Patent Laying-Open No. 58-60678 discloses a tough boron nitride-based superhigh pressure sintered material for a cutting or wear-resistant tool which is composed of cubic boron nitride and a binder phase consisting of a nitride and a carbide of at least one of Ti, Hf and Si so that a layer of a nitride having an average thickness of 0.1 to 2 .mu.m encloses the cubic boron nitride.
Japanese Patent Laying-Open No. 5-186844 discloses a tough boron nitride-based superhigh pressure sintered material for a cutting or wear-resistant tool which is composed of cubic boron nitride and/or wurtzite boron nitride and a binder phase consisting of at least one of the metals belonging to the groups 4a, 5a and 6a of the periodic table, carbides, nitrides, oxides and borides of Al, Si, Fe, Ni and Co, an oxide and a nitride of a rare earth metal and a solid solution thereof, and Fe, Ni and Co so that a layer of at least one of nitrides and borides of Ti, Hf, Zr, Mo, Al and Si and solid solutions thereof having an average thickness of 0.5 to 90 nm encloses the cubic boron nitride and/or the wurtzite boron nitride.
The high-pressure phase boron nitride-based sintered bodies described in the aforementioned Japanese Patent Laying-Open Publications are obtained by coating high-pressure phase boron nitride particles with binders and sintering the same. Thus, these sintered bodies can reduce the occurrence of unsintered parts, and are improved in wear resistance and chipping resistance.
In each of these sintered bodies, however, the coating binder is made of ceramic which is a brittle material, and this ceramic is so thermally and chemically stable that it causes insufficient reaction with the high-pressure phase boron nitride to result in insufficient adhesion strength between the coating binder and the high-pressure phase boron nitride particles. Thus, the coating binder is readily chipped and separated from the high-pressure phase boron nitride particles when the coated particles are mixed with another binder or pressurized by a superhigh pressure sintering device or a hot press. In the high-pressure phase boron nitride-based sintered body employing such powder materials, the high-pressure phase boron nitride particles are frequently in contact with each other, which leads to sufficient homogenization of the sintered body structure.